Java: check if arraylist is part of fibonacci sequence

My assignment for school is to implement a method that checks if a given

ArrayList

is part of the Fibonacci sequence.

The array must not be empty and must be bigger than 3.

I understood that I have to check if one number of the array and the next one are part of the Fibonacci sequence, however I have a lot of trouble with it since you're supposed to accept the array if it's any part of the sequence and not just from the start.

e.g.:

0 1 1 2 3 5

will be accepted as well as

2 3 5 8 13 21

.

This is my code so far. I know it's very flawed but i really have no clue how to move on.

public class ArrayCheck {
/**
* Tests if the given array is a part of the Fibonacci sequence.
*
* @param arr array to be tested
* @return true if the elements are part of the fibonacci sequence
*/
public boolean isFibonacci(ArrayList<Integer> arr) {
//check if array exists
if(arr.size() == 0)
return false;

This is bad, as this mean 0 1 1 5 which is not part of the sequence will return true.

What you need to do is split this into two tasks:

Find the first relevant number in the sequence (i.e. if the array starts with 7, you know this isn't a part of the sequence; alternatively, if it starts with 8, you know you need to start checking from 8 onward).

Once you've found the "start", simply check that the rest of the array follows the Fibonacci rule. you'll need to manually verify the first two items.

public boolean isFibonacci(ArrayList<Integer> arr) {
if (arr.size() < 3){
return false;
}
/** find if the first element is part of the sequence: **/
int fib1 = 0;
int fib2 = 1;
while (fib1 <= arr.get(0)) {
int tmp = fib1 + fib2;
fib1 = fib2;
fib2 = tmp;
}
if (fib1 != arr.get(0)) {
// first element is not part of Fibonacci sequence
return false;
}
if (fib2 != arr.get(1)) {
// the first two elements are not part of the Fibonacci sequence
return false;
}
/*** now simply verify that the rest of the elements uphold the rule:
each element is the sum of the two previous ones: **/
for(int i=2; i < arr.size(); i++) {
// make sure there are no negatives in the array:
if (arr.get(i) < 0)
return false;
if (arr.get(i) != (arr.get(i-1) + arr.get(i-2))
return false;
}
//everything checks out okay - return true:
return true;
}

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